The Chelmsford (Mass.) Water District serves 26,000 customers on a $5.3 million annual budget. Managers expect 2,300 photovoltaic panels to supply more than half the energy their 3-mgd treatment plant needs every year. Photos: MassDEP, Division of Municipal Services

The American Recovery and Reinvestment Act of 2009 couldn't have come at a better time for state and local authorities in Massachusetts.

Two years earlier, the state had pledged to increase solar capacity at public facilities by up to 132 megawatts. At the same time, a water district was examining ways to conserve resources. The two goals dovetailed perfectly to qualify for the stimulus requirement that states devote at least 20% of their water and wastewater revolving-loan allocations to energy and water efficiency, green infrastructure, and environmental innovation.

Today, the Chelmsford Water District's 3-mgd treatment plant in Crooked Spring has one of New England's largest solar photovoltaic arrays. The recently completed 485-kilowatt solar field is expected to supply about 55% of the plant's annual power requirements, but on any given day it may produce more than the plant needs. When that happens, the local electricity utility applies a credit to the district's bill.

DESIGN-BUILD FOR TIGHT DEADLINES

Electricity burns up to 40% of a treatment plant's operating budget. In 2007, to help local governments reduce pollution and save money, EPA and the Massachusetts departments of Environmental Protection and Energy Resources launched a pilot planning initiative to evaluate renewable and energy-efficiency options.

Thinking back to Chelmsford Water District's operational evaluation, Environmental Compliance Manager Todd Melanson says “we could've made small changes like changing all of our light bulbs. But we knew that wouldn't get us where we wanted to be: reducing energy consumption 25% to 30% over the next 10 to 15 years.”

Melanson and his colleagues had been attending quarterly energy roundtable discussions the state was hosting with the University of Massachusetts (UMass) Lowell. There they learned that although wind energy is more economical (see “Sea breeze” on page 35 of the July issue), most of the state's inland sites don't have the recommended 13-mph minimum velocity to be technically feasible. But Massachusetts does have sufficient insolation — the sun's energy incidence on a region in a calendar year — for photovoltaic power.

In February 2009 — with passage of the economic stimulus just around the corner — the district engaged a graduate student from UMass Lowell's Renewable Energy Engineering Department to analyze shading and evaluate potential panel layouts at its mostly flat, 30-acre Crooked Spring site. The plant treated 596 mgd total in 2009 — well below its peak hydraulic capacity of 4.5 mgd — and its 150-hp, high-head pumps consumed 1.2 million kilowatt hours (kWh) of electricity at a cost of $127,000.

The report concluded that ground-mounting panels at 22° F could produce 40% of peak hourly demand. That was enough to convince Melanson and the state that solar power was feasible.

The stimulus gave Massachusetts $186 million for water and wastewater revolving loans. After deciding to use one-third to fully fund the 14 green projects the state's pilot program had identified, water authorities began looking for projects that could meet “shovel-ready” deadlines.

The district's project was already planned and approved, so authorities fast-tracked the project using the two-year-old Chapter 25A procurement law, which allows for design-build.

IT'S ALL ABOUT COMPROMISES

The art and science of solar photovoltaic power

Components: An array of cells, called panels, and an inverter.

Operation: The panels convert the sun's radiation to direct current (DC), which the inverter converts to alternating current (AC). Whenever the panels produce more than a plant needs, the excess electricity is routed to the grid and the customer's bill is credited. When the sun's not shining, the grid supplies power conventionally.

Considerations: The more radiation they receive, the more power the panels produce.

Ideally, panels should face due south at an angle roughly equal to the latitude. To maximize production in the summer, when electricity's more expensive but the sun's farthest from the earth, a shallower angle increases the normal incident of radiation; i.e., the amount of sunlight that hits the panel perpendicular to the plane of the face of the panel.

In reality, the tilt and azimuth are usually a compromise based on available area, site conditions, shading issues such as trees, power poles, and other panels, and architectural concerns.

Another factor is response curve. Different types and sizes of panels react differently with different sun conditions and positions.

The “perfect” installation would be in the desert facing due south tilted at the site's latitude. But you'd have to somehow transport that power to the consumer, who's probably nowhere near the installation. That's how you end up with rooftop arrays in Ohio that are facing southwest and tilted at 10 degrees.